fixed bug when images are not correctly specified by wordpress

app/src/main/java/fr/forum_thalie/tsumugi/ui/news/NewsAdapter.kt

 // but without Picasso (just fetching image by myself.)
 class ImageGetterAsyncTask(
     private val context: Context,
-    private val source: String,
+    private val source: String?,
     private val levelListDrawable: LevelListDrawable
 ) :
     AsyncTask<TextView?, Void?, Bitmap?>() {